1. Field of the Invention
The present invention relates to a distortion measuring apparatus for measuring variations in distortion generated in an optical fiber using a BOTDR (Brillouin Optical Time Domain Reflectometry) method, and further relates to the distortion measuring apparatus in which an accuracy of the measurement can be improved.
2. Description of the Related Art
In order to monitor anomalies such as a collapse or a landslide of a monitored object such as a ground such as a slope face or a fabric such as an inner face of a tunnel, there are distortion measuring apparatuses for measuring distortions generated in an optical fiber which is laid on the monitored object as a distortion generated in the monitored object using an OTDR method using Brillouin scattered light (BOTDR method).
This type of distortion measuring apparatus comprises a sensor cable which is fixed on the monitored object, and a BOTDR measuring instrument which emits pulsed light into the sensor cable and measures the Brillouin scattered light returned from the sensor cable. The distortion generated in the monitored object is measured by the distortion measuring apparatus in accordance with the following process.
When the pulsed light which is emitted by the BOTDR measuring instrument is inputted in the optical fiber which is provided in the sensor cable, the light is scattered at a suitable position in the optical fiber and the Brillouin scattered light is generated. In this Brillouin scattered light, the light which is scattered backwardly in the optical fiber (backward scattered Brillouin scattered light) reversely travels in the optical fiber and returns to the BOTDR measuring instrument.
A frequency of the light is varied in compliance with the generation of the Brillouin scattering. When the frequency of the inputted pulsed light is denoted as xcexd0, and a central frequency of the spectrum of the Brillouin scattered light is denoted as xcexd, a shift xcex4 of the frequency of the spectrum of the Brillouin scattered light is denoted as xcex4=xcexdxe2x88x92xcexd0. The shift xcex4 of the frequency of the spectrum of the Brillouin scattered light can also be denoted as a function between the distortion xcex5 of the optical fiber at the position in which the scattering is generated and temperature T. When the temperature T of the optical fiber is constant, the shift xcex4 of the frequency is denoted as a linear function toward the distortion xcex5 of the optical fiber as shown in a following equation 1.
xcex4=xcex4(0)+Cxc2x7xcex5xe2x80x83xe2x80x83Equation 1
In the equation 1, a symbol xcex4(0) denotes the shift of the frequency when the distortion of the optical fiber is zero, and a symbol C denotes a proportion constant which is logically determined or determined based on actual measurement.
Furthermore, when the shift of the frequency and the distortion of the optical fiber at a certain point of time are respectively denoted as xcex41 and xcex51, and the shift of the frequency and the distortion of the optical fiber at an another point of time are respectively denoted as xcex42 and xcex52, the variation of the shift of the frequency xcex42xe2x88x92xcex41 and the variation of the distortion xcex52xe2x88x92xcex51 has a relationship as shown in a following equation 2.
xcex42xe2x88x92xcex41=Cxc2x7(xcex52xe2x88x92xcex51)xe2x80x83xe2x80x83Equation 2
Therefore, a relative variation of the distortion of the optical fiber from a certain point of time can be calculated by detecting the backward scattered Brillouin scattered light, measuring the shift of the frequency, and analyzing the result of the measurement. Consequently, the variation of the distortion of the monitored object can be estimated based on the distortion of the optical fiber.
Furthermore, in the BOTDR method, a distance from the BOTDR measuring instrument to the position in which the Brillouin scattering is generated can be calculated from the amount of time from inputting the pulse light into the optical fiber to returning the light which was scattered to the BOTDR measuring instrument. Consequently, the variation of the distortion of the optical fiber can be measured as the position in which the variation of the distortion of the optical fiber is generated, and therefore, the position in which the distortion was generated on the monitored object can be estimated.
Normally, measured data of the distortion obtained by the BOTDR method is described as a graph (BOTDR waveform) in which a horizontal line denotes the distance from the BOTDR measuring instrument, and a vertical line denotes the distortion of the optical fiber. In this graph, it is necessary to determine the above-described xcex4(0) in order to find an absolute amount of the distortion of the optical fiber. However, it is not easy to determine the value of xcex4(0), and therefore, the relative variation of the optical fiber which is based on the relative level which is assigned to zero is generally used as the value described along the vertical line of the BOTDR waveform.
However, when the variations of the distortion of the optical fiber in the sensor cable are successively measured several times using the above-described distortion measuring apparatus, there are cases in which the BOTDR waveform drifts up and down. As a result, the true variation of the distortion of the optical fiber cannot be distinguished from the apparent variation of the distortion of the optical fiber which is generated by the drift of the BOTDR waveform, and therefore, the distortion cannot be measured accurately. In particular, when the BOTDR measuring instrument is switched ON/OFF or when a plurality of channels in which each channel is connected with the sensor cable are connected with the BOTDR measuring instrument and are switched, relatively large drifts tend to be generated.
An object of the present invention is, therefore, providing an apparatus and a method in which an accurate measurement of the variation of the distortion of the optical fiber can be performed by excluding the apparent variation of the distortion of the optical fiber which is generated by the drift of the BOTDR waveform.
The above-described object is achieved by using a distortion measuring apparatus comprising a sensor cable and a reference fiber which is connected with the sensor cable in series, and by subtracting the apparent variation in the distortion of the reference fiber from a measured variation of the distortion of the sensor cable in order to correct the measured variation of the distortion of the sensor cable.
It is preferable that the reference fiber be housed into an thermostatic chamber and that the temperature of the reference fiber be maintained at a predetermined value within a range of 10 to 40xc2x0 C. with an error within xc2x12C.
It is further preferable that the reference fiber be bundled in a free-coil shaped having a diameter of 20 to 30 cm and be placed on a stationary place for preventing an appearance of a lateral pressure in order to avoid a further variation of the distortion of the reference fiber generated by an external force.
It is further preferable that the length of the reference fiber be not less than 20 meters in order to distinguish the position of the reference fiber in the BOTDR waveform.
Furthermore, when the reference fiber is provided between the sensor cable and the BOTDR measuring instrument of the distortion measuring apparatus, if a plurality of channels in which each channel is connected with the sensor cable connected with the BOTDR measuring instrument, a plurality of reference fibers provided for these sensor cables can be housed into the same thermostatic chamber, and therefore, the distortion measuring apparatus can be provided economically.